1. Field of the Invention
The present invention relates to a method and an apparatus for correcting defects in a photomask, and more particularly to a method and an apparatus for correcting so-called clear defects in a photomask, which are defects formed by the loss of the light-shielding film of the photomask used in the manufacture of semiconductors or of liquid crystal displays.
2. Description of the Related Art
Conventional methods to correct clear defects in a photomask include a method to form a light-shielding film over clear defective parts of the photomask, which is disclosed in the Japanese Patent Laid-open No. 1989-124236. According to this conventional correcting method, a photomask is placed in an input gas atmosphere, and the input gas is locally decomposed on the photomask by irradiating the clear defective parts of the photomask with a laser beam to form a light-shielding film over the clear defective parts. In order to satisfy the requirements of tight adhesion to the substrate and light-shielding, both essential for a photomask, a metallic film of Cr or the like is deposited by laser chemical vapor deposition (CVD). This conventional correcting process uses as input gas a mixture of chrome carbonyl gas (Cr(CO).sub.6) and argon gas, resulting in the formation of a film excelling in light-shielding and adhesion to the substrate.
Since this conventional clear defect correcting method for photomasks involves deposition of only a metallic light-shielding film over the substrate to correct clear defects, the reflectance of the light-shielding film deposited over the defective area is as high as about 40%. As a result, when the photomask is to be checked for any defect on the basis of the intensities of scatterred and reflected light from the substrate, the effect of light reflected from this light-shielding film having a high reflectance causes the previously defective parts already corrected by the deposition of the light-shielding film to be mistaken for uncorrected defects. At every step of inspection, these parts mistaken for defects have to be confirmed not to be defective, leading to a serious problem in the management of photomask manufacturing.